The invention relates to a flush-back filter for liquid media, in particular for liquid fuels, having a filter inlet, a filter outlet and a filter housing with a sludge drain, a support cage penetrated from the interior to the exterior by the medium to be filtered firmly fixed in the filter housing for a least one filter element arranged at its exterior, having at least two groups of slot-type windows arranged on the support cage, whereby the windows within each group run in parallel to one another and are separated from one another by parallel narrow webs of the support cage and having a flush-back device on the inside of the support cage movable by means of an actuator or drive and draining off the filter residues to the sludge drain, said flush-back device overlapping upon its movement the windows of the support cage with a slit aperture or the like in succession and thereby progressively clearing the filter area during filter operation using the differential pressure between the filter outlet and the sludge drain.
Flush-back filters of this type, used preferably in fuel systems for drives such as in particular ship engines are known from the DE-PS No. 14 36 267. The cylindrical support cage used here has slot-type windows closely running in parallel to each other which extend respectively over an arcuate circumference of approx. 80.degree. to 85.degree. and which are separated from each other by narrow arcuate webs. Four window groups in all are provided over the cylindrical circumference of the support cage which comprise in each case a plurality of windows vertically one over the other. With the upward stroke of the plate-like slide valve, which is actuated with the help of a pressure loaded movable piston, in each case four windows on a common radial plane of the support cage are overlapped simultaneously by a peripheral groove arranged at the periphery of the slide valve with the result that the filter is cleaned over a partial area by the flush-back filtrate and is freed from all adhering impurities, said filter extending essentially over the entire scope of the support cage. Backflushing over the relatively large partial areas requires appropriately large flushing volumes and leads furthermore to a relatively large pressure drop on the discharge side of the filter and so, too, within the system.
With the known flush-back filters the backflushing operation occurs furthermore only upon upstroke of the slide valve. At the beginning of the downstroke of the slide valve the connection to the sludge outlet is automatically cut off by means of a closing or plugging member which is arranged on an adjustable rod bearing the slide valve with no-load stroke, penetrating the support cage, said adjustable rod supporting the adjustable piston guided at its upper end in a cylinder.
From the DE-AS No. 23 27 532 a flush-back filter is also known whereby the cleaning of the cylindrical filter jacket is effected with the help of several cleaning nozzles distributed over the height of the filter insert which are rotatably arranged in the filter insert and operate alternately. These cleaning jets in the form of air jets can be opened respectively via an own sludge drain to the free atmosphere, whereby the reverse flow of cleaned fluid is effected through the filter jacket and the respective opened cleaning nozzle. Thereby the cleaning device is controlled in such a way that during one cleaning cycle respectively only one single cleaning nozzle is opened to the pressureless sludge drain. In this way the filter jacket can be successfully cleaned over relatively small partial areas and with correspondingly lower flush-back volumes at a decreased pressure drop in the system. However, the constructional expenditure for the drive and control of the various cleaning nozzles is considerable here.